According to the state of the art, the pressure in a hydraulic circuit is controlled according to need in a gearbox, especially in an automatic gearbox for a motor vehicle. While the pressure level in the hydraulic circuit for the lubricating oil supply of the gearbox components can be kept low, the pressure must be strongly increased during shifting operations, for example, to be able to fill the control elements rapidly.
To control the pressure in the hydraulic circuits, pressure controllers that regulate downstream check valves for the clutch operation are normally used. Control of the downstream check valves takes place within the pressure controller by means of a proportional magnet that consists, among other things, of a magnetic core, a magnetic coil, and a magnetic anchor. The coil flow is controlled in proportion with the output variable force by means of the proportional magnet; the magnetic anchor, and hence the downstream check valves, are controlled in accordance with the coil flow. From the characteristic magnetic force flow curve of the pressure controller resulting therefrom are generated the pressure flow curves (P/I-curves) desired for the clutch adaptation in an electrohydraulic control of automatic gearboxes.
From the German patent application DE 100 34 959 A1 of the applicant is known a proportional pressure control valve having a valve part with inlet and outlet openings and at least one closing mechanism for controlling an opening, as well as a magnetic part with a magnetic core, a magnetic coil, and a magnetic anchor arranged to be displaceable. The anchor coacts herein with an actuating element that actuates the spherical closing mechanism. In addition, the hydraulically acting cross section of the opening is determined essentially by the opening length, the opening diameter, and the diameter of the part of the actuating element that penetrates into the opening, wherein the ratio of opening length to opening diameter is less than 2.0.
Another proportional pressure control valve is known from WO 98/48332 of the applicant. It has at least two opening phases, wherein two phases are variable and mutually coupled by mechanical or hydraulic action according to the hydraulic half bridge principle and are provided as inlet and outlet openings of a control pressure chamber.
From DE 100 03 896 A1 of the applicant is also known a pressure controller, which adjusts a downstream check valve according to need by means of a fixed proportional magnet, a magnetic coil, a movable magnetic anchor, and a specific control system and controls in this way the pressure in the hydraulic circuit. Initially, the characteristic pressure flow curve has a continuously rising gradient. In practice, however, the gradient at the onset of the pressure flow curve is very high for the operational requirements, so that the pressure flow sensitivity of the pressure control needs improvement for the gear change requirements. An increase in the shifting quality can effect an improvement in sensitivity, in particular at a low load, that is, at low pressures.
To improve shifting quality, the applicant has proposed within the scope of DE 102 44 527 to realize a load-dependent adaptation of the P/I-curve of a proportional pressure control valve by means of a proportional magnet with two mutually independent controllable gaps. Accordingly, the first gap part is a plunging phase known from the state of the art that is controlled by means of an induced magnetic field. This magnetic field is generated by an electric current in the magnetic coil and runs over the magnetic anchor, the housing, and the magnetic core located in the interior of the magnetic coil.
Within the scope of the solution described in DE 102 44 527 is provided an axial separation between the magnetic anchor and the magnetic core by means of a non-magnetic non-stick disc in such a way that the magnetic field can project beyond a gap of the magnetic anchor. At this point, the magnetic field generates a magnetic force that acts on the magnetic anchor and moves the same in an axial direction in accordance with the magnetic force. This makes possible continuous control of the magnetic anchor, or of a control element connected to the magnetic anchor, via an anchor rod by means of continuous electric activation of the proportional magnet or the magnetic coil. Furthermore, a targeted weakening of the magnetic field can be achieved by means of a second gap. The second gap is realized by using a magnetic core that consists of at least two parts, wherein a first part is securely connected to the housing and arranged so that is concentrically and axially displaceable around the anchor rod, which is securely connected to the magnetic anchor. The second part of the magnetic core is arranged to be coaxially, concentrically, and axially displaceable around the anchor rod, and in contrast to the first part, is configured to be axially movable, so that an adjustment of the gap between the two magnetic core parts is made possible. This means that the magnetic resistance dependent from the gap width can be controlled so that the gradient of the P/I-curve is lower in particular with low current values than in comparable solutions, but is still not optimal.
In all the solutions according to the state of the art, the gradient of the P/I-curve is relatively steep, especially at lower current values, so that improvements of the shifting comfort can be achieved.
It is an object of the invention to disclose a proportional pressure control valve for controlling the pressure level in a hydraulic circuit, especially in a hydraulic circuit of a gearbox of a motor vehicle, which improves upon the state of the art. Especially the P/I-curve should have a very flat rising gradient or a high pressure/current resolution in part at low current values.
This object is attained by means of a proportional pressure control valve.